Envelope elimination and restoration (EER) principle is known in the art. For example, Raab (U.S. Pat. No. 6,256,482 B1) discloses a power-conserving drive-modulation method for EER transmitter, wherein an envelope baseband component and a phase modulation (PM) radio-frequency (RF) component of a modulated signal are separately amplified and then combined in a signal for transmission. The major advantage of the EER transmitter is that the RF PM signal is amplified by highly efficient nonlinear RF amplifiers and the last RF amplifier is amplitude-modulated by the envelope signal, thereby restoring the modulation at the output of the transmitter. In Raab, the envelope and phase components are separated by analog means using a limiter for removing the envelope component such that only the PM component is present at the limiter output and an envelope detector is used to detect the baseband component from the original modulated RF waveform. The envelope and phase components can also be generated by a digital signal processor (DSP), which gives better performance over the analog counterpart. As shown in FIG. 1, the envelope component is generated by an envelope modulation module 12 in the DSP and D/A converted. After frequency filtering by a low-pass filter F1, the envelope component is changed into a pulse-width modulation (PWM) format in a PWM modulator, which feeds a switch-mode class-D or class-S amplifier A1. The digital PWM signal from the amplifier A1 is converted into an analog signal by a low-pass filter F3. The switch-mode amplifier has higher efficiency than traditional analog amplifiers. The phase component directly modulates a frequency synthesizer (PLL, F2, VCO). After being amplified by a driver A2, the phase component is amplitude-modulated in a class-E power amplifier A3.
Alternatively, when the modulation bandwidth is high and the required PWM frequency is high, which increases losses in the switching transistors, it is advantageous to use an analog amplifier A4 to provide the envelope waveform for amplitude modulation, as shown in FIG. 2.
High-efficiency RF modulation can also be found in McCune (U.S. Pat. No. 6,377,784), which discloses a method of reducing the spread between a maximum frequency of a desired modulation and the operating frequency of a switch-mode DC-DC converter.
Switch-mode envelope modulation is within the limits of present technology of EDGE (Enhanced Data Rates for GSM Evolution). However, the efficiency of switch-mode envelope modulation for use in WCDMA (Wideband Code-Division Multiple Access) is poor due to the high switching frequency needed to achieve required modulation bandwidth in WCDMA. Thus, it is advantageous and desirable to provide an envelope restoration transmitter, which is capable of efficiently transmitting signals in the GSM/EDGE mode as well as in the WCDMA mode.